Switching between subjective and objective modes is the essence of the scientific modus operandi. Not many people seem to appreciate that. Science is all about riding two horses, maybe not in concert, but certainly alternately - and knowing when to switch from one to the other.

Tuesday, January 12, 2010

Thanks for visiting this post, penned some 6 years ago. It's still attracting 15-20 visits per day! (Maybe it's that graphic below of those thermally-agitated gas molecules that attract visitors, rather than the wordy science per se - yup, I'm a down-to-earth realist).

Before proceeding onto your perilous journey into the intricaies of how molecules - light versus heavy- respond (or don't) to the Earth's gravitational field, please be advised that this investigative blogger/retired scientist has made other contributions to knowledge (or as some unkind folk would say - fantasy). There are two in particular I wish to flag up:1. Stonehenge. I believe it to have been designed initially for SKY BURIAL, or as I prefer to call it, AFS (avian-facilitated skeletonization).

2. The Shroud of Turin. I believe it to be a medieval fake, consistent with the radiocarbon dating (1260-1390). I believe it was intended to reproduce what Joseph of Arimathea's 'fine linen' - used simply to transport the crucified Jesus from cross to tomb - would have looked like 13 centuries later if it had captured a body imprint in SWEAT and BLOOD. I have demonstrated in my own home how the enigmatic body image could have been imprinted onto wet linen using an adult male volunteer. The lucky fellow was first smeared with vegetable oil, then coated with a dusting of white flour. Heating in a bread-making oven or similar (the linen, not the bloke) would have produced the sepia body image, later attenuated to its final 'ghostly' appearance by vigorous washing with soap and water.

Thanks to kinetic energy, there will be no appreciable 'unmixing' - or even partial stratification,- at least under normal g values. That's the case even if one type of molecule is much heavier than the other

The question in the title was inspired by a comment on a site I visit a lot, one to which I've posted a lot in nigh on three years. More about that later. Suffice it to say that I shall reply to here, rather than there, and attempt to place a link on the other site. There will be no more posting to that site until such time as its moderation policies are given a thorough overhaul! Nuff said for now...

The question was essentially this: given that we all know that petrol fumes sink to the ground at a filling station, why doesn't CO2 - which we also know is denser than air - also settle at ground level? Why are we not suffocated by the stuff - or does it only come up to ankle or knee level?

Imagine one were to trap gases inside balloons - one for hydrogen, one for oxygen, one for nitrogen, one for carbon dioxide - and then release them. The four balloons would behave exactly as the questioner supposes. The hydrogen balloon would quickly ascend, the CO2 balloon would rapidly descend, and the nitrogen and oxygen balloons would probably hover or sink slowly - due mainly to the weight of the balloon rubber - not the contents. The relative densities of hydrogen :nitrogen: oxygen: air::carbon dioxide are approximately 1 : 7 : 8 : 7.2 : 22. Gases lighter than air rise, those heavier than air sink. No surprises there.

When petrol fumes are released, they too sink quickly, at least to start with. A typical molecule in petrol is one of the isomeric octanes, general formula C8H18, with a relative vapour density of 57 - some 4 times greater than air.

But the petrol fumes would not stay for very long at ground level. Convection current carry them upwards, and gaseous diffusion would cause mixing with air even without convection. That's because gas molecules are in a state of constant motion, colliding with other molecules, millions of times a second, causing them gradually to diffuse ("spread") in all directions. The fumes gradually spread into all the space available - which could be a jar, a garage, a hangar, the entire atmosphere. Once the space is evenly occupied, the molecules then show no tendency to unmix. Why not? Answer: because the 1g force that acts on all molecules in air at sea level is insufficient to overcome the kinetic forces due to collision between molecules. Put more simply - a molecule that gets a strong bump from below will be knocked upwards, against the weaker force of gravity.

This is true for g=1, but is not true for progressively higher g forces.

Here's an example - always a a controversial one. Enrichment of the fissile uranium isotope U-235 needed for atomic power stations OR Hiroshima-type A bombs, requires separation from the more abundant U-238. This can be accomplished in gaseous diffusion plants, or in centrifuges that generate an intense g force. Either process requires that solid metallic uranium first be converted to the gaseous uranium hexafluoride (UF6).

There is a well known experiment that is done in schools, at any rate, those that still have a fume cupboard, to demonstrate that dense gases and/or vapours gradually diffuse to fill the space available, and then do not subsequently unmix from air.

One places of few drops of elemental bromine, Br2, a fuming red liquid in the lower jar, which is separated from the upper jar by a glass divider. One waits for the lower jar to fill completely with red-brown fumes. One then removes the separator. The fumes gradually fill both jars evenly, despite bromine vapour being 5 times denser than air.

There's a variant on the experiment that I devised while teaching to demonstrate the petrol vapour effect. One places a jar of bromine on top, and then removes the divider. Most of the bromine fumes sink immediately into the lower jar, behaving as if they were enclosed in a balloon. But the fumes then gradually diffuse back upwards to produce the same end-result as before.

The short term behaviour of the petrol fumes is called a bulk phase effect. It's the temporary behaviour of heavy molecules in close proximity, which behave briefly as if enclosed in a balloon. But once diffusion has caused mixing of heavy molecules with the lighter molecules of nitrogen and oxygen, unmixing does not occur at normal values of g.

Think then of a gas before diffusion and mixing as a kind of ghost fluid, with its own density and buoyancy characteristics. In fact, while the term "fluid" in everyday life is synonymous with "liquid", in physics it applies to both gases and liquids. But a gas loses the distinguishing characteristics of its original 'fluidity' once it's had time to spread sufficient for its own kind of molecules to become separated and irreversibly mixed with other kinds of molecules.

So there is an effect (of sorts) that relates to molecular mass ("heaviness"),
even if there is no unmixing as such. Does that contradict anything that precedes
it in this posting? Discuss. :-)Here's my own interpretation, for what it's worth, of the grading by molecular size/mass with increasing altitude (added September 17 2014)?

As one gets higher, the air thins (this being due to decreasing gravitational pull on everything, gases included, such that most gas is held relatively close to the Earth's surface (a few tens of kilometres). But another effect can then operate that discriminates according to molecular mass. It's to do with the average spacing between molecules and their average speed (best measured we're told as the root mean square velocity). As the molecules become further apart they can travel further using their own intrinsic motion before colliding with another to be deflected off in a different direction, impeding upward progress.

But a light molecule travels faster at a given temperature than a heavier one. It's one of the givens of kinetic molecular theory. It explains why hydrogen gas diffuses faster than carbon dioxide. Thus there is a greater probability that a lighter molecule like hydrogen will be able to cross a given transient empty space faster than a heavier onebefore the gap, so to speak, closes up. Ipso facto, light molecules have a greater probability of "winning the race" to the top of the Earth's atmosphere. But having got there they will find they are still held by the Earth's gravitational field albeit much weaker than at ground level, unless exceptionally light, like hydrogen and/or helium atoms which we are told can and do escape from the Earth's atmosphere, leaking off into interplanetary space.

Summary: what's operating is
not settling out of heavier molecules in response to gravity. It's the speedier motion in an otherwise unfavoured direction (upwards) of molecules that are LIGHTER and thus able to DIFFUSE faster! (They would diffuse anyway, whether or not a gravitational field was present, so gravitation becomes a secondary consideration).

Note: I'm pushing the limits of my physics in offering the above interpretation. If folk find it faulty, and/or can offer a better explanation for the sorting-by-size effect with altitude, then please feel free to comment. However, I do not consider the phenomenon is serious enough to challenge the generalization that gases do not spontaneously and efficiently unmix of their own accord, at least in a natural gravitational field around a planet-size object. Random molecular motion with constant collisions always ensures that molecules will never completely unmix, while accepting there can be concentrating effects of the kind described under normal or elevated g forces.

The takeaway message is the inverse square law that relates molecular mass to average speed at particular temperatures. A nitrogen molecule, N2, can be calculated to have an average speed of 508 metres per second at 28 degrees C (301K), though it would have to be in a perfect vacuum to be able to traverse that distance. A molecule that was 4 times as heavy would travel at half that speed, a half being the inverse square root of 4. A molecule that was half as heavy would travel at, er, darn, where did I put that pencil? Off the top of my head I think the answer is root 2 times as fast. That's approx 1.4 times as fast, i.e. 40% faster or thereabouts. A hydrogen molecule, H2, has 1/14th the mass of a nitrogen molecule, N2, so would travel (1/root 1/14) i.e. approx 3.75 times faster.

Addendum, 18th September 2014

I'm not sure I've adequately explained the difference between having a gas confined within a balloon or not, especially as regards the 'thought experiment' of taking the balloon away to watch the disappearance of density characteristics, "sinking" etc,

Here's a home-made diagram that will be used to describe my current (and still evolving) thinking on the subject:

Brace yourselves. More to come

First, look at the right-half of the balloon, and imagine the left were the same, i.e. an intact envelope enclosing gas all the way round. In that situation, the normal laws of buoyancy would apply, which as a revision exercise I shall now show in three diagrams (A-C) filched from the internet, of increasing detail and complexity.

Diagram A:

Diagram A above shows an object (only) partly immersed in a fluid, which is subject to two forces: gravity, pulling it down, and "buoyancy" pushing it up. But what is the nature of the buoyancy force? The diagram does not explain. Let's look at another which does.

Diagram B:

This diagram shows an object fully immersed in the fluid (I wish that Diagram A had too, but beggars/filchers can't be choosers). Note that the fluid exerts pressure on the object, that the pressure acts in all directions, that being the nature of pressure as a result of billions of random molecular collisions per second that have no single directionality. Note the upwards pointing arrow in the middle. Why is the nett force upwards ("buoyancy"). Again, the diagram does not explain. For that we need to go to the next diagram.

Diagram C:

What this diagram shows is the imbalance of forces acting on the immersed object. The pressure at the bottom (pressure being force per unit area) is greater than at the top, and indeed greater than at all points between top and bottom. In other words, the nett force is upwards. the nett upwards force is called the UPTHRUST.

For the object to float, the upthrust needs to be greater than the weight of the object in air. For the object to sink, the upthrust must be less than the weight of the object. Upthrust can be measured as the weight of fluid displaced (handy for calculation, while not giving insights into the mechanism of upthrust which as explained is due to increasing pressure with depth producing an imbalance of forces between highest and lowest points).

Now let's return to our sealed/soon to become leaky balloon and compare with the three diagrams above:

Hopefully, dear reader, you can guess what is coming. While that balloon is intact, with the same kind of molecules all packed together, exerting their particular density characteristics, whether smaller or greater than the surrounding air, then the balloon goes up or down, following the laws of buoyancy, the gas behaving just the same as any other fluid.

However, imagine that balloon envelope suddenly becoming permeable, with gas escaping and mixing, then one no longer has a homogeneous fluid of characteristic density and buoyancy. As the escaping molecules begin to mix with the surrounding molecules of air, then the bulk effects disappear, the molecules then behaving more or less independently from their neighbours, now increasingly different. What matters now are not the original bulk properties that respond to gravity, and/or the contingent pressure differences that depend on gravity, but the behaviourof the individual particles comprising the originally-enclosed gas, which is now determined by their intrinsic molecular speed, which is in turn a function of temperature, kinetic energy, mass and velocity, summed up in the term diffusibility.

Interestingly, there's a transition period between release from a confining receptacle and complete mixing (whether by slow diffusion, or aided by air currents etc) when the body of gas is still sufficiently discrete to continue behaving as a fluid. Some of us recall the demo experiment in school chemistry labs where teacher takes a jar full of CO2 gas and "pours" it over a candle or lit Bunsen burner, the flame being instantly extinguished in both cases.

Come to think of it, might the idea that CO2 "sinks and suffocates" be based on reports where the gas has been released from underground, say, or under water (as in the 1986 Lake Nyos disaster in Cameroon) , in both instances in regions of volcanic activity where the gas has vented from subterranean magma, and then flowed as a 'fluid' for a considerable time before there was time for mixing to occur?

Lake Nyos disaster, 1986

But that is not CO2 settling out from a mixture, needless to say, which by now I hope is an idea that no one will entertain. The lethal invisible blanket of gas issuing from the Earth's bowels has been able to retain its density characteristics in the period between initial venting and subsequent mixing with other gases in the atmosphere, notably oxygen and nitrogen.

Update: September 22 2014

I discovered today why this posting attracts far more visitors each day than any of my other postings, despite having been written some 5 years ago. Assuming that most visitors were finding it via their search engines, I tried entering strings of search terms that correspond with the title, and then whittling them down to a core set. To my surprise, I find that one has simply to enter (CO2 heavy) and this posting tops the list of returns! It's clearly achieved that virtuous circle, aka critical mass, where its present prominence helps ensure continuing prominence!

Never one to rest on laurels, I've been making some additions by way of afterthoughts, and picking up on points that others have raised elsewhere, notably on science discussion forums where the content comes chiefly in serial additions from the participants themselves, starting with someone's primer question. In fact there's just such a forum that arrived three years after this one, posing essentially the same question, and is now third in my list of Google returns.

Looking at the points made, I'm more than ever convinced that I was right to raise the question, since clearly there is some confusion in people's minds (as there was initially in my own) as to the importance or otherwise of bulk density v molecular weight where the behaviour of 'heavy' gases is concerned, before and after mixing in a gravitational field.

At the risk of giving this post an intimidating length, I might try supplying my own answers to some of the points raised. Or there again, it might be wise to create a separate follow-up post so as not to overload this one.

Oh, and here's a link to a climate change sceptic, maybe denialist even, who seems to think that CO2 is too heavy to get into the upper atmosphere. In fact,the faux science gets worse as one reads on, much worse.

Sample: (my italics)

"How mad with power does a group of people become that they now want to
control, CO2, a naturally occurring colorless, odorless, incombustible
gas formed during respiration, decomposition of organic substances, volcanic emissions, decay of plant and soil organic matter? A gas
that was intelligently designed to be heavier than air for a purpose.
How crazy is that?

I invite him to read this posting and reconsider.

October 5, 2014:

Have just come across this blog, with a brief mention at the end of a delightful reductio ad absurdum argument. Look for the term "layer cake atmosphere".

October 6, 2014

And here's a must-see paper from a kindred spirit (an Italian caver at Turin University) who unlike myself has the maths to support the theory. The title says it all. Click to enlarge if you wish to read the abstract.

Note the date of first publication: April, 2009, i.e. some 9 months before I penned this posting. But I'm not a plagiarizer, honest, no, really, HONEST, not having spotted this paper until just a few days ago.

New addition

November 8th 2014

Here's an additional 'thought experiment' using CO2 and helium filled balloons to demonstrate that once mixed, gases do not unmix.

I composed it yesterday as a new blog posting, and intended to add a brief summary here. Being somewhat busy right now, here's a cut-and-paste of the entire posting, which I shall endeavour to prune when I've some fee time.
------------------------------------------------------------------------------------------

Still they keep coming, to a posting I did nearly 5 years ago.

Posted on this site, Jan 12, 2010

It wasn't as if CO2 and its behaviour in a
gravitational field was a burning issue at the time. In fact that
posting was a side-issue from my then preoccupation with the climate
change/global warming debate (which I was glad to withdraw from, given
the brow-beating denialist tendency on MSM forums).

It was provoked as I recall by a barmy comment to the effect that CO2 had been made "heavy" by a Benign Presence (Gaia? Guardian angel in attendance?)
so as to stay at lower altitudes, feeding our plants, not reaching
higher altitudes where it might cause mischief!!!

Here's
a snapshot from my sitemeter, showing that 11 of the last 20 visits
have been to that posting (13 is a more typical average).

The red tags are mine, pointing to the CO2 posting

Quite why it still gets all the attention I haven't a clue. Maybe it's because I'm a Londoner
it's currently the second listing one sees if entering (CO2 heavy)
into Google. Sometimes it's the first he says in a rare moment of
modesty-bypass.

To
be honest, I've been somewhat embarrassed that a instant tutorial
should now be seen as the first or second stop on a simple non-technical
web search. So much so that I gave it a spring clean this last
September, adding bits here and there to strengthen the case (as I'm
only 99.9% certain about my conviction that normal g forces - from the
Earth's own pull - are insufficient to cause unmixing of CO2 and
stratification, ONCE THE GAS has diffused and mixed with the nitrogen
and oxygen of air).

With that as preamble, folk should
perhaps understand why I'm back again, still fine-tuning, still
whittling down that stubborn 0.1% of doubt.

What I wish
to describe now is a thought experiment. (Yes, I know it should or
could have been a real one, but if thought experiments were good enough
for Albert Einstein then they are good enough for me - that's my story
and I'm sticking to it).

It's a development of the
'teaching aid' in the original posting, which began with a brief look at
the behaviour of gases trapped inside balloons, where they do indeed
show their heavier or lighter-than-air characteristics. That's before
the gases have escaped from their balloons, then diffused and mixed with
air and lost their ability to "sink", settle out, stratify, call it
what you wish.

Last night I had a brainwave.
Why not keep the gases trapped inside their balloons, and allow them to
mix by diffusion (which may take a few minutes, possibly a lot of
minutes for totally even and homogeneous mixing, but mix they will, such
is the nature of gaseous diffusion).

How might the behaviour of the balloons compare before and after mixing? Let's do that thought experiment.

Before
opening the valve: the two attached balloons may ascend, descend, or
stay put, depending on the relative size of the two balloons, and the
average density of the two gases compared to that of the surrounding
air. If the average density is less, the system ascends etc. (And it
won't matter a jot whether the gases are separate or pre-mixed or post-mixed for that to be true - important for what follows).

But
one thing's for certain. The two balloons will remain oriented with
respect to each other, as in the diagram, with the blue helium balloon
on top. That's because it always experiences more upthrust than the red
balloon, displacing a greater volume of air for a given weight of
enclosed gas. If one attempts to alter the stacking geometry, the system
will self-correct when released.

Now let's
picture what happens if the valve is opened, or the fusible wax plug is
melted, allowing the two gases to mix. One could allow mixing by
diffusion only, which means a lot of waiting. Alternatively one can
speed up mixing by inverting the balloons as shown below.

xx

The lighter gas helium, now underneath, will ascend;
the heavier CO2 will descend, and being a two-way countercurrent system
there will be faster mixing than if it were by diffusion alone. A series
of inversion, re-righting, re-inversion etc should result in a
homogeneous distribution of gases between the two balloons.

What happens when one releases the two (still attached) after mixing?

Answer:

If the original system ascended, so will the new one.

If the original system descended, so will the new one.

If the original system was perfectly balanced, neither ascending nor descending, so will the new one. But there will be a difference. Both balloons are now equally buoyant
(or non-buoyant, depending on the proportions of the two gases). So
there will be no tendency for one balloon to be above the other. In
other words, the two balloons can adopt any configuration through 360
degrees (with a slight tendency maybe for the smaller balloon to 'lead
the way' if rising or sinking, due to aerodynamic differences).

So the two attached balloons might go up, go down, or stay put, looking like this:

or like this:

or any angles of rotation in between.

One
thing's for certain. Restoring the original configuration to blue on
top, red underneath will not cause reversion to the original
self-correcting orientation, since that would require that the gases
unmix, with CO2 going back into the red balloon, helium going back on
top.That as we've seen. simply does NOT happen at normal values of g.
Which is where we came in...

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About Me

Colin Berry, aka sciencebod, is a retired PhD researcher/teacher/academic who has worked in industry, medical schools, schools, food and biomedical research (mainly in the UK, but also in W.Africa and the United States). He's best known for his work on RESISTANT STARCH, recently described as "the trendiest form of dietary fibre".
See also his specialist Shroud of Turin blog on www.shroudofturinwithoutallthehype.wordpress.com
with over 200 postings to date.

Create one's own blog (age, class, gender no barrier)

It's really quite straightforward. All one has to do is to click on the photograph with that nice young man. One can then be part of the frightfully interesting Blogger community in just a couple of jiffs.

Acknowledgment

What's the latest on the LHC?

LHC gets warning system upgrade : BBC 28 September 2009

Self-organization

From wiki entry on SELF ORGANIZATION: "As a result, processes considered part of thermodynamically open systems, such as biological processes that are constantly receiving, transforming and dissipating chemical energy (and even the earth itself which is constantly receiving and dissipating solar energy), can and do exhibit properties of self organization far from thermodynamic equilibrium."

How far away should your off-licence be for a bottle of wine to be energy-neutral?

What do these two have in common?

Answer: both arrived in this world about the same time. Sir Isaac Newton was born on 4th Jan 1643 (new style*). The Taj Mahal had a 20 year gestation period, centred on approximately the same year. Click on piccy for an older post .* Or Christmas Day, 1642, depending which dating system one uses.

Is interstellar space travel feasible?

The nearest star (more correctly, star system, since it's 3 stars, a binary and a smaller satellite star) is Alpha Centauri. The average distance from Earth is 4.3 light years. Suppose technology allows us one day to achieve an interstellar cruising speed of half the speed of light. A comfortable acceleration of g (simulating Earth's gravity) would take a year, with another year to slow down comfortably. The entire journey from Earth would take a minimum of 10 years approximately. Having arrived at one's destination, it would take 4.3 years to send a radio postcard (" Hello Mum and Dad. Have arrived safely, and am now looking for a habitable planet. Am hoping it's hiding behind Proxima. Have looked everywhere else... Would die for some Cheddar cheese... ")

Watch this space

It's a cheap and cheerful form of transcendental meditation.(experimenting with settings, actually)

What causes weather?

Could you answer that question in just 7 words, ie " weather is due to...? Need some help, " Weather is due to t- - u - - - - - - h - - - - - - o - t - - E- - - -'s s - - - - - - ." The National Curriculum (England and Wales) does have its uses, but there are many more such simple principles, expressed in a minimum of words, that could be usefully incorporated.

"Had there been a Beginning (there wasn't, as it happens), there would initially have been complete Nothingness. But just as Nature abhors a vacuum, it's totally gutted at the thought of Nothingness. I mean to say - how far does Nothingness extend, assuming it has one of more dimensions? It can't extend for an infinite distance, since that would be a physical impossibility. Nothingness, to avoid having infinite reach, coils up on itself to acquire finite dimensions. In so doing, it becomes Somethingness, which has a spring-like potential energy - the total energy in fact of the Universe.

From that potential energy, present in what we now call space, or space-time, which is anything but empty, is spawned all sub-atomic particles - both matter and antimatter. When those particles collide, they mutually annihilate to create photons.

The reverse can also happen under extreme conditions - two photons can collide to create matter and anti-matter. It is potential energy in the spring-coiled Universe that is our "Dark Energy. It may or may not have mass depending on conditions.

A moment when it has no mass is the instant of the Big Bang. Let me briefly explain. An oscillating universe switches between Big Bang and Big Crunch. With the latter gravitation pulls everything into a super blackhole which then becomes a singularity - a massively dense point in space-time.

What prevents it becoming infinitely small - a physical impossibility? Answer: friction. As the sub-atomic plasma contracts and grinds, heat is generated which cannot escape - being a black hole. The temperature rises, ie particles in the plasma move faster and faster. When they reach their maximum velocity - the speed of light- all particles are suddenly transformed into photons, which as we know have no true mass(at least, no rest mass: any mass they have is purely relativistic due to their speed).

Once the entire Universe is a super-concentration of photons, all the gravitational forces in the singularity collapse to zero, or nearly so, and the entire thing blows apart - a new Big Bang, to create yet another cycle (inflation, Big Crunch, implosion etc). The Big Bang creates not just sub-atomic particles - from photon-photon collisions, but space-time itself. To reiterate: that space-time is always suffused with the stored potential energy of our curled-up dimensions (Dark Energy)."